This invention relates to an air intake meter which operates on the principles of a Karman vortex flowmeter for measuring the amount of intake air of an internal combustion engine. More particularly but not exclusively, it relates to an intake air meter for an internal combustion engine of an automobile having a circuit for compensating for fluctuations in the pressure in the main intake air passageway due to the pulsing action of individual cylinders.
A fuel injection system for an internal combustion engine requires constant monitoring of the amount of intake air. A Karman vortex flowmeter is particularly suitable for measuring the intake air of an internal combustion engine of an automobile because it has no moving parts and therefore has good vibration resistance.
FIG. 1 is a block diagram of a conventional intake air meter employing a Karman vortex flowmeter which was disclosed in Japanese Patent Publication No. 58-56415. This intake air meter has a Karman vortex flowmeter 1 comprising a vortex shedder 2 including an obstruction which is disposed in the center of an unillustrated main intake air passageway of an internal combustion engine. Intake air 14 which enters the main intake air passageway flows past the vortex shedder 2, which sheds a Karman vortex street 3. An ultrasonic transmitter 4 and an ultrasonic receiver 5, which confronts the ultrasonic transmitter 4, are disposed on opposite sides of the main intake air passageway downstream of the vortex shedder 2. The ultrasonic transmitter 4 is driven by an oscillator circuit 6, and generates ultrasonic waves which propagate across the main intake air passageway and are received by the ultrasonic receiver 5. In crossing the main intake air passageway, the ultrasonic waves are phase modulated by the Karman vortex street 3. The output of the ultrasonic receiver 5 is input to a first waveform shaping circuit 8 which amplifies and shapes it and inputs it as a first input signal to a phase comparator 9. The output of the oscillator circuit 6 is input to a voltage-controlled phase shifter 7 which produces an output signal whose phase is shifted from that of the output from the oscillator circuit 6 by an amount which is controlled by the voltage of the output from a loop filter 10. The output from the voltage-controlled phase shifter 7 is input as a second input signal to the phase comparator 9, which produces an output corresponding to the phase difference between the two input signals. This output is input to the loop filter 10, which removes unwanted frequency components from the output of the phase comparator 9. The voltage-controlled phase shifter 7, the phase comparator 9, and the loop filter 10 thus form a phase locked loop. The output of the phase comparator 9 is also input to a low-pass filter 11 which removes the carrier frequency component from the output of the phase comparator 9.
The voltage-controlled phase shifter 7 maintains the high-frequency stability of the output signal from the oscillator circuit 6 while controlling only its phase shift. The characteristics of the loop filter 10 in the phase locked loop are chosen to have adequate speed to follow the modulation angular frequency of the signal which is phase modulated by the Karman vortex street 3. The output of the loop filter 10, which is used as a phase demodulated output, varies so as to make the output of the voltage-controlled phase shifter 7 synchronous with the output from the ultrasonic receiver 5. The phase synchronization angle of the phase locked loop is determined by the characteristics of the phase comparator 9 and the loop filter 10. By using a recently-developed phase comparator integrated circuit, phase synchronization angles of 0, .pi./2, .pi., etc. can be easily attained.
Due to the overlapping of the operation of the intake valves of an internal combustion engine, particularly in a multicylinder engine, the flow of intake air fluctuates periodically. At the instant when an unillustrated intake valve opens, the pressure within the intake manifold suddenly decreases. When the throttle valve of the engine (not illustrated) is nearly completely open, the sudden drop in pressure is transmitted past the throttle valve to the main intake air passageway in which the vortex shedder 2 is located. Therefore, the pressure within the main air intake passageway fluctuates, causing the Karman vortex street 3 to be generated in an irregular manner. As a result, the output signal of the low-pass filter 11, shown by the solid line in FIG. 2a, contains a time-varying, pressure-dependent component, indicated by the dashed line in the same figure, and the average amplitude of the output of the low-pass filter 11 fluctuates along with the pressure-dependent component. The output from the low-pass filter 11 is normally shaped to produce square waves based on the time the output crosses two inversion levels 12. The resulting shaped output is shown in FIG. 2b. Ideally, each peak in the output of the low-pass filter 11 should correspond to one square wave in the shaped output of FIG. 2b, but due to the pressure-dependent component, the output of the low-pass filter 11 does not always cross the inversion levels 12, and a portion of the desired waveform is missing, as shown by the dashed lines in FIG. 2b. Because of the missing square waves, the number of Karman vortices generated can not be correctly counted and the intake air rate cannot be accurately measured.